linux-kernel - Re: [v2 PATCH] fs/proc: task_mmu.c: don't read mapcount for migration entry

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Message-ID: <f7f82234-7599-9e39-1108-f8fbe2c1efc9@redhat.com>
Date:   Wed, 26 Jan 2022 12:38:04 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     Yang Shi <shy828301@...il.com>, kirill.shutemov@...ux.intel.com,
        willy@...radead.org, akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [v2 PATCH] fs/proc: task_mmu.c: don't read mapcount for migration
 entry

On 26.01.22 12:29, Jann Horn wrote:
> On Wed, Jan 26, 2022 at 11:51 AM David Hildenbrand <david@...hat.com> wrote:
>> On 20.01.22 21:28, Yang Shi wrote:
>>> The syzbot reported the below BUG:
>>>
>>> kernel BUG at include/linux/page-flags.h:785!
>>> invalid opcode: 0000 [#1] PREEMPT SMP KASAN
>>> CPU: 1 PID: 4392 Comm: syz-executor560 Not tainted 5.16.0-rc6-syzkaller #0
>>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
>>> RIP: 0010:PageDoubleMap include/linux/page-flags.h:785 [inline]
>>> RIP: 0010:__page_mapcount+0x2d2/0x350 mm/util.c:744
>>> Code: e8 d3 16 d1 ff 48 c7 c6 c0 00 b6 89 48 89 ef e8 94 4e 04 00 0f 0b e8 bd 16 d1 ff 48 c7 c6 60 01 b6 89 48 89 ef e8 7e 4e 04 00 <0f> 0b e8 a7 16 d1 ff 48 c7 c6 a0 01 b6 89 4c 89 f7 e8 68 4e 04 00
>>> RSP: 0018:ffffc90002b6f7b8 EFLAGS: 00010293
>>> RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
>>> RDX: ffff888019619d00 RSI: ffffffff81a68c12 RDI: 0000000000000003
>>> RBP: ffffea0001bdc2c0 R08: 0000000000000029 R09: 00000000ffffffff
>>> R10: ffffffff8903e29f R11: 00000000ffffffff R12: 00000000ffffffff
>>> R13: 00000000ffffea00 R14: ffffc90002b6fb30 R15: ffffea0001bd8001
>>> FS:  00007faa2aefd700(0000) GS:ffff8880b9d00000(0000) knlGS:0000000000000000
>>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>> CR2: 00007fff7e663318 CR3: 0000000018c6e000 CR4: 00000000003506e0
>>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>>> Call Trace:
>>>  <TASK>
>>>  page_mapcount include/linux/mm.h:837 [inline]
>>>  smaps_account+0x470/0xb10 fs/proc/task_mmu.c:466
>>>  smaps_pte_entry fs/proc/task_mmu.c:538 [inline]
>>>  smaps_pte_range+0x611/0x1250 fs/proc/task_mmu.c:601
>>>  walk_pmd_range mm/pagewalk.c:128 [inline]
>>>  walk_pud_range mm/pagewalk.c:205 [inline]
>>>  walk_p4d_range mm/pagewalk.c:240 [inline]
>>>  walk_pgd_range mm/pagewalk.c:277 [inline]
>>>  __walk_page_range+0xe23/0x1ea0 mm/pagewalk.c:379
>>>  walk_page_vma+0x277/0x350 mm/pagewalk.c:530
>>>  smap_gather_stats.part.0+0x148/0x260 fs/proc/task_mmu.c:768
>>>  smap_gather_stats fs/proc/task_mmu.c:741 [inline]
>>>  show_smap+0xc6/0x440 fs/proc/task_mmu.c:822
>>>  seq_read_iter+0xbb0/0x1240 fs/seq_file.c:272
>>>  seq_read+0x3e0/0x5b0 fs/seq_file.c:162
>>>  vfs_read+0x1b5/0x600 fs/read_write.c:479
>>>  ksys_read+0x12d/0x250 fs/read_write.c:619
>>>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>>>  do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
>>>  entry_SYSCALL_64_after_hwframe+0x44/0xae
>>> RIP: 0033:0x7faa2af6c969
>>> Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 11 15 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
>>> RSP: 002b:00007faa2aefd288 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
>>> RAX: ffffffffffffffda RBX: 00007faa2aff4418 RCX: 00007faa2af6c969
>>> RDX: 0000000000002025 RSI: 0000000020000100 RDI: 0000000000000003
>>> RBP: 00007faa2aff4410 R08: 00007faa2aefd700 R09: 0000000000000000
>>> R10: 00007faa2aefd700 R11: 0000000000000246 R12: 00007faa2afc20ac
>>> R13: 00007fff7e6632bf R14: 00007faa2aefd400 R15: 0000000000022000
>>>  </TASK>
>>> Modules linked in:
>>> ---[ end trace 24ec93ff95e4ac3d ]---
>>> RIP: 0010:PageDoubleMap include/linux/page-flags.h:785 [inline]
>>> RIP: 0010:__page_mapcount+0x2d2/0x350 mm/util.c:744
>>> Code: e8 d3 16 d1 ff 48 c7 c6 c0 00 b6 89 48 89 ef e8 94 4e 04 00 0f 0b e8 bd 16 d1 ff 48 c7 c6 60 01 b6 89 48 89 ef e8 7e 4e 04 00 <0f> 0b e8 a7 16 d1 ff 48 c7 c6 a0 01 b6 89 4c 89 f7 e8 68 4e 04 00
>>> RSP: 0018:ffffc90002b6f7b8 EFLAGS: 00010293
>>> RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
>>> RDX: ffff888019619d00 RSI: ffffffff81a68c12 RDI: 0000000000000003
>>> RBP: ffffea0001bdc2c0 R08: 0000000000000029 R09: 00000000ffffffff
>>> R10: ffffffff8903e29f R11: 00000000ffffffff R12: 00000000ffffffff
>>> R13: 00000000ffffea00 R14: ffffc90002b6fb30 R15: ffffea0001bd8001
>>> FS:  00007faa2aefd700(0000) GS:ffff8880b9d00000(0000) knlGS:0000000000000000
>>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>> CR2: 00007fff7e663318 CR3: 0000000018c6e000 CR4: 00000000003506e0
>>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>>>
>>
>> Does this point at the bigger issue that reading the mapcount without
>> having the page locked is completely unstable?
> 
> (See also https://lore.kernel.org/all/CAG48ez0M=iwJu=Q8yUQHD-+eZDg6ZF8QCF86Sb=CN1petP=Y0Q@mail.gmail.com/
> for context.)

Thanks for the pointer.

> 
> I'm not sure what you mean by "unstable". Do you mean "the result is
> not guaranteed to still be valid when the call returns", "the result
> might not have ever been valid", or "the call might crash because the
> page's state as a compound page is unstable"?

A little bit of everything :)

> 
> In case you mean "the result is not guaranteed to still be valid when
> the call returns":
> We're just collecting stats for userspace, and by the time we return
> to userspace, the numbers will be outdated anyway, so that doesn't
> matter.

Yes, it's a snapshot.

> 
> In case you mean "the result might not have ever been valid":
> Yes, even with this patch applied, in theory concurrent THP splits
> could cause us to count some page mappings twice. Arguably that's not
> entirely correct.

Yes, the snapshot is not atomic and, thereby, unreliable. That what I
mostly meant as "unstable".

> 
> In case you mean "the call might crash because the page's state as a
> compound page could concurrently change":

I think that's just a side-product of the snapshot not being "correct",
right?

> As long as we have our own mapping of the page, the page can't be
> split, so this patch fixes that problem.
-- 
Thanks,

David / dhildenb